Stem cells are often defined as self-renewing and multipotent, with the ability to generate diverse types of differentiated cells. As such, they show promise in the treatment of neurological and corporal disorders (also referred to as neurological and corporal “deficits”), or any loss or diminishment of tissue function due to age, disease, trauma or other factor. However, such treatments have faced significant hurdles that have yet to be substantially overcome.
NSCs and Neurological Deficits
Because an important focus of stem cell replacement therapies has been neurological disorders, neural stem cells, and particularly fetal neural stem cells, have been a major research target. During development of the central nervous system (CNS), multipotent neural stem cells (MNSCs), also known as multipotent precursor cells (MPCs), or tissue-specific neural stem cells (NSCs), proliferate, giving rise to transiently dividing progenitor cells that eventually differentiate into the cell types that compose the adult brain, including neurons, astrocytes and oligodendrocytes. NSCs have been isolated from several mammalian species, including mice, rats, pigs and humans. See, e.g., International Application, Publication Nos. WO 93/01275, WO 94/09119, WO 94/10292, WO 94/16718 and Cattaneo et al., 1996, Mol. Brain. Res. 42: 161-66. NSCs from the embryonic and adult rodent central nervous system (CNS) have been isolated and further propagated in vitro in a variety of culture systems. See, e.g., Frolichsthal-Schoeller et al., 1999, NeuroReport 10: 345-351; Doetsch et al., 1999, Cell 97: 703-716. NSCs from the human fetal brain have been cultured using serum-free medium supplemented with epidermal growth factor (EGF) and/or basic fibroblast growth factor (bFGF). See, e.g., Svendsen et al., 1998, J. Neurosci. Meth. 85: 141-152; Carpenter et al., 1999, Exp. Neurol. 158: 265-278. NSCs cultured utilizing these serum-free, mitogen-supplemented methods generally form substantially undifferentiated, clustered aggregates. Upon removal of the mitogen(s) and provision of a substrate, these neural stem cells differentiate into neurons, astrocytes and oligodendrocytes.
While the synaptic connections involved in neural circuits are continuously altered throughout the life of the individual, due to synaptic plasticity and cell death, neurogenesis (the generation of new neurons) was thought to be complete early in the postnatal period. The discovery of MNSCs in the adult brain (see, e.g., Alvarez-Buylla et al., 1997, J. Neurobiol 33: 585-601; Gould et al., 1999, Science 286: 548-552) has significantly changed the theory on neurogenesis, as the presence of MNSCs in the adult brain suggests that regeneration of neurons can occur throughout life. Nevertheless, age, physical and biological trauma or neurodegenerative disease-associated loss of brain function, herein referred to as a “neurological deficit,” can far outweigh any potential restorative effects due to endogenous neurogenesis. As a result, up-regulated or stimulated proliferation of endogenous MNSCs as well as transplantation of MNSCs are potentially valuable treatments for those suffering from the loss of, or loss of appropriate, brain function due to age, physical and biological trauma or neurodegenerative disease (i.e., a neurological deficit). No such treatments are known in the art.
Due to the advancing average age of the population, and concomitantly increased incidence of neurological deficit that accompanies advancing age, treatment of neurodegenerative diseases has become a major concern. Such diseases, including Alzheimer's disease, Huntington's chorea and Parkinson's disease, have been linked to neuronal degeneration at specific locations in the brain, leading to the inability of the brain region to synthesize and release neurotransmitters that are vital to neuronal signaling.
Neurodegeneration also encompasses many conditions and diseases, age-related or not, that result in neuronal loss. These conditions include CNS trauma, such as ischemia (stroke) and epilepsy, as well as diseases that result in neuronal loss, including amyotrophic lateral sclerosis and cerebral palsy.
Many such neurological deficits are localized to particular regions of the brain. Degeneration in a brain region known as the basal ganglia can lead to diseases with varied and different cognitive and motor symptoms, depending on the exact location of the lesion. The basal ganglia consists of many separate regions, including the striatum (which consists of the caudate and putamen), the globus pallidus, the substantia nigra, substantia innominata, ventral pallidum, nucleus basalis of Meynert, ventral tegmental area and the subthalamic nucleus.
Degeneration in the basal ganglia can lead to motor deficits. For example, Huntington's chorea is associated with degeneration of neurons in the striatum, which leads to involuntary jerking movements. Degeneration of a small region called the subthalamic nucleus is associated with violent flinging movements of the extremities in a condition called ballismus, while degeneration in the putamen and globus pallidus are associated with a condition of slow writhing movements or athetosis. In Parkinson's disease, degeneration is seen in another area of the basal ganglia, the substantia nigra par compacta. This area normally sends dopaminergic connections to the dorsal striatum, which are important in regulating movement. Therapy for Parkinson's disease has centered upon restoring dopaminergic activity to this circuit.
Alzheimer's disease patients exhibit a profound cellular degeneration of the forebrain and cerebral cortex. Further, a localized area of the basal ganglia, the nucleus basalis of Meynert, appears to be selectively degenerated. This nucleus normally sends cholinergic projections to the cerebral cortex that are thought to participate in cognitive functions including memory.
The objective of most CNS therapies is to regain the particular chemical function or enzymatic activity lost due to cellular degeneration. Administration of pharmaceutical compositions has been the main treatment for CNS dysfunction though this type of treatment has complications, including the limited ability to transport drugs across the blood-brain barrier, and drug-tolerance acquired by patients to whom these drugs are administered for long periods.
Transplantation of multipotent stem cells may avert the need not only for constant drug administration, but also for complicated drug delivery systems necessitated by the blood-brain barrier. In practice, however, significant limitations have been found in this technique as well. First, cells used for transplantation that carry cell surface molecules of a differentiated cell from a donor can induce an immune reaction in the recipient, a problem that is exacerbated by the physical damage caused by injection of cells directly into the affected area of the brain. In addition, the neural stem cells must be at a developmental stage where they are able to form normal neural connections with neighboring cells. For these reasons, initial studies on neurotransplantation centered on the use of fetal cells.
Mammalian fetal brain tissue has proven to have reasonable survival characteristics upon immediate transplantation. Increased survival capability of fetal neurons is thought to be due to the reduced susceptibility of fetal neurons to anoxia compared to adult neurons. An additional factor favoring survival of fetal cells is their lack of cell surface markers, whose presence may lead to rejection of grafted tissue from adults. However, although the brain is considered an immunologically privileged site, some rejection of even fetal tissue can occur. Therefore, the ability to use heterologous fetal tissue is limited by tissue rejection and the resulting need for immunosuppressant drug administration.
The use of large quantities of aborted fetal tissue presents other difficulties as well. Fetal CNS tissue is composed of more than one cell type, and thus is not a well-defined tissue source. In addition, it may be unlikely that an adequate and constant supply of fetal tissue would be available for transplantation. For example, in the treatment of MPTP-induced Parkinsonism, tissue from as many as 6 to 8 fetuses can be required for successful implantation into the brain of a single patient. There is also the added problem of the potential for contamination during fetal tissue preparation. Since this tissue may already be infected with a bacteria or virus, expensive diagnostic testing is required for each fetus used. Even comprehensive diagnostic testing might not uncover all infected tissue. For example, there can be no guarantee that a sample is HIV-free, because antibodies to the virus are generally not present until several weeks after infection.
In addition to fetal tissue, there are other potential sources of tissue for neurotransplantation, including cell lines and genetically engineered cell types, but both sources have serious limitations. Cell lines are immortalized cells that are derived, inter alia, by transformation of normal cells with an oncogene or by the culturing of cells in vitro with altered growth characteristics. Moreover, adverse immune response potential, the use of retroviruses to immortalize cells, the potential for the reversion of these cells to an amitotic state, and the lack of response by these cells to normal growth-inhibiting signals make such cell lines sub-optimal for widespread use.
Another approach to neurotransplantation involves the use of genetically engineered cell types or gene therapy. However, there still exists a risk of inducing an immune reaction with these cells. In addition, retrovirus mediated transfer may result in other cellular abnormalities. Also, cell lines produced by retrovirus-mediated gene transfer have been shown to gradually inactivate their transferred genes following transplantation and further may also not achieve normal neuronal connections with the host tissue.
Currently available transplantation approaches suffer from significant drawbacks. The inability in the prior art of the transplant to fully integrate into the host tissue, and the lack of availability of suitable cells in unlimited amounts from a reliable source for grafting are significant limitations of neurotransplantation. Studies utilizing intra-tissue injection of dissociated and partially differentiated NSCs have shown little promise (see, e.g., Benninger et al., 2000, Brain Pathol. 10: 330-341; Blakemore et al. 2000, Cell Transplant 9: 289-294; Rosser et al., 2000, Eur. J. Neurosci. 12: 2405-2413; Rubio et al., 2000, Mol. Cell. Neurosci. 16: 1-13). The results have generally been poor because, among many considerations, the dissociation of clusters of NSCs is known to cause immediate senescence of NSCs and increase the vulnerability of NSCs in culture. See, e.g., Svendsen et al., 1998, J. Neurosci. Meth. 85: 141-152. Further, regardless of adverse immune responses provoked by foreign tissue being introduced into the brain, the trauma caused by the physical introduction of cells directly into the damaged area can induce the recruitment of immune cells by the host that can eliminate the transplanted cells. Thus, significant problems with the use of NSCs to ameliorate neurological deficits remain. As described herein, neurological deficits also include non-brain tissues such as, for example, the eye and spinal cord.
A “corporal deficit” is a disorder caused by a wide variety of diseases and injuries, resulting in trauma, malfunction, degeneration or loss of muscle such as, for example, cardiac muscle due to myocardial infarction. Other examples include malfunction, degeneration or loss of other cells and tissues apart from those discussed in the neurological deficit section above such as, for example, internal organs. For example, liver function can be adversely affected by, among other things, disease (e.g., cirrhosis or hepatitis), trauma or age. The problems described above in using NSCs to remedy neurological deficits of the brain also apply to neurological deficits in other tissues, such as the eye, and corporal deficits.
There exists a need in the art for improved methods for increasing the number of multipotent cells in an animal and thereby increasing the reservoir of remedial capacity conferred by multipotent stem cells in tissues. There exists a need to stimulate proliferation, migration or both proliferation and migration of endogenous and exogenously introduced mammalian multipotent stem cells in vivo as well as mammalian multipotent stem cells in vitro. There exists a need for cells stimulated to proliferate, migrate or both proliferate and migrate, as well as pharmaceutical compositions for treating a neurological deficit or corporal deficit comprising such stimulated cells. Further, there exists a need in the art for methods of administration of such cells stimulated to proliferate, migrate or both proliferate and migrate and pharmaceutical compositions thereof. Still further, there exists a need for methods for treating an animal having a neurological or corporal deficit.